1. Field of the Invention
The present invention relates to a molybdenum board which has excellent strength at high temperatures, and a process of manufacturing the same.
2. Description of the Prior Art
Molybdenum is used as a material of heat treatment jigs such as furnace heaters or heat treatment boats which are used at high temperatures since molybdenum has a high melting point and good heat-resistance properties. However, if a heat treatment jig obtained by working molybdenum board is used under conditions of high temperatures which are around the recrystallizing temperature of molybdenum or higher and involve heating/cooling, recrystallization occurs during the use of the jig, and deformation or cracking may occur due to thermal fatigue or creep. As time elapses, such deformation or cracking progresses to a degree, in case of using molybdenum to form furnace heaters, to cause abnormal contact with each other which causes short-circuiting and melting off of the heaters. Then, the temperature profile of the furnace heater becomes abnormal; local high temperatures or disconnection occurs. The furnace heater cannot then serve its intended purpose. Further, if heat treatment jigs such as sintering boats and mounting plates of sintered materials used in automatic lines for sintering oxides or carbides such as uranium dioxide (UO.sub.2) at a temperature of about 1,500.degree. C. or higher are deformed to a substantial degree, the sintered materials may fall down from the boats or plates. In an extreme case, the molybdenum boards contact each other and the sintered materials cannot be mounted thereon, thus, they become to be unable to accomplish their intended purposes. Further, when the thermal conductivity of the compounds to be sintered is different from that of molybdenum, the molybdenum jig is sometimes broken due to a stress generated in each heat treatment between the surface on which sintered material is mounted and other surfaces of the jig.